The following account of the prior art relates to one of the areas of application of the present invention, optical communications systems.
Loss Reduction:
A planar optical branching component, such as a power Y-splitter, often plays a fundamental role in simple as well as complicated designs. The function that a Y-splitter performs is to divide the incoming signal into typically two signals of equal magnitude (50%:50%). It should be noted, that even though we in this text focus on the so-called Y-splitters, the presented idea can be used in branching components that divide the incoming signal(s) into more than just two output waveguides as well as into varying splitting ratios, not just 50%:50%.
Often it is of interest to be able to divide one or more signals into a large number of signals. To achieve this goal typically a so-called splitter tree is used where a number of fundamental splitters are concatenated. A 1-4 splitter (tree) is then realized by concatenating two 1-2 splitters to the two outputs of a first 1-2 splitter. Furthermore, a 1-8 splitter is realized by concatenating four 1-2 splitters to the 1-4 splitter (one 1-2 splitter to each of the four outputs). In this way it is possible to realize 1-2N splitter-trees using only the simple 1-2 splitter. 2-2N can be realized using a similar approach where a 2-2 branching component (e.g. a coupler) is followed by two separate 1-2N−1 splitter trees.
Typically it is desirable that the splitter distributes the incoming signal equally between the output waveguides and with minimum optical loss. To achieve the goal of equal splitting, the individual splitters need to divide the incoming signals equally, i.e. x %:x % where x is as close as possible to 50.
If a splitter divides the signal equally and without loss, i.e. x=50, the intensity of the signal in the two output waveguides will be Poutput=−10*log(½)=3.01 dB lower than the original signal. For a loss-less 1-4 splitter the power in the four output waveguides will be Poutput=2*3.01=6.02 dB lower than the original signal, and for a 1-16 splitter the output power will be Poutput=4*3.01=12.04 dB than the original signal. Thus for a 1-16 splitter the theoretical minimum reduction in the power level in an equally splitting component will be 12.04 dB.
There is no such thing as a loss-less passive optical components since there will always be coupling losses, propagation losses, radiation losses etc.
These loss factors can be minimized by proper choice of waveguide cross-sectional geometry, refractive index of the core material and cladding materials, as well as by choosing suitable curve-forms for the component layout. By suitable choices we managed to fabricate low-loss 1-16 components having typical values of insertion loss in the neighbourhood of 13.5 dB which is 13.5−12.04=1.46 dB above the theoretical minimum. Assuming that the extra loss is equally distributed among the four splittings in the 1-16 splitter tree the loss per fundamental splitter is 1.46 dB/4=0.365 dB (neglecting the coupling loss from the fibre to the chip). In order to come near the theoretical minimum we thus need to reduce the excess loss per splitter by a few tenths of a dB.
U.S. Pat. No. 5,745,618 discloses a 1-P power splitter comprising an input waveguide and P output waveguides that are all coupled to a slab waveguide in the form of a planar area, which is large compared to the area of an individual waveguide and designed to support light wave transmission between the input and output waveguides. The power splitter further comprises a transition region immediately adjacent to the slab waveguide which comprises a number of silica paths (e.g. 30) that transversely intersect the output waveguides. The silica paths are generally parallel to each other and have widths that progressively decrease as they become further away from the slab waveguide.
The present application discloses an optical branching component and a method of reducing the loss of optical power in branching components, in particular 1-2N splitters, which makes possible the fabrication of e.g. 1-16 (N=4) splitters having an insertion loss of approximately 12.3-12.5 dB (typically).
Stress Relieving:
The present invention further deals with the issue of reducing stress induced polarization effects and stress induced tilting of the cores in planar optical components, e.g. waveguide couplers. Since the top-cladding introduces a non-symmetric strain field across closely spaced waveguides, measures have to be taken to minimize these effects.
One way to reduce this non-symmetric behaviour is to place the waveguide cores of the coupler as far apart as possible to approach an isolated waveguide situation. This way each waveguide core in the coupler will see a quasi-symmetrical surrounding and the strain field will be more uniform across the waveguide core. By placing the waveguide cores far apart in the coupler region, the length of the coupler device will be significantly larger. Therefore this solution is not very suitable for compact device design.
Other means of reducing this effect is by using a polymer over-cladding that is heat-treated at low temperatures (<˜300° C.). Because of the relatively low process temperature, the thermally induced stress effects will be smaller and consequently the stress levels lower. Polymer top-cladding has an inherent reliability problem and is therefore generally not used in commercial products.
Gap-filling:
The present invention further deals with the issue of gap-filling, which for planar waveguide fabrication is a technological challenge. Since most of the deposition processes are of a planar type, special measures have to be taken to fill high-aspect-ratio trenches (e.g. trenches that have a height to width ratio larger than 2 where the height dimension is taken in a direction of growth of the planar process and the width dimension is taken perpendicular thereto). This can be done by adding e.g. boron and phosphorus to the glass whereby the flowing temperature can be reduced to typical anneal temperatures used in planar waveguide fabrication. The reflow-properties of the glass is, however, very dependent on the structures that have to be covered, e.g. a directional coupler. A directional coupler may be used either as an individual component in itself, or as a part in a larger functionality. A directional coupler consists of two separate waveguides which, over a distance known as the coupling length (LCR), are closely spaced (cf. FIG. 21).
If the two cores pertaining to the two separate waveguides are sufficiently close, the exponentially decaying tail of the optical field in the first waveguide core may be able to reach into the core of the second waveguide core. Being in the second waveguide core, the field pertaining to the first waveguide core is creating a polarization of the atoms in the core medium which in turn generates a new optical field in the second core. The greater the magnitude of the field pertaining to waveguide 1 in waveguide 2, the greater is the polarization and hence the faster the transfer of energy, which translates to a shorter length for full coupling. In this way the energy in the field in the first waveguide core can be gradually transferred to the second waveguide core. As the field decays exponentially outside the core regions of either of the waveguides, it is necessary that the two waveguide cores are closely spaced if good coupling and hence a short coupling length is to be achieved. If the distance between the two waveguide cores increases, the distance along the length in which a certain percentage of the energy from the field in the first waveguide is coupled to the field in the second waveguide increases exponentially. The smaller the components, the more components per unit area or wafer may be implemented, which—for directional couplers—requires that the waveguide cores in the coupling region be closely spaced.
In a directional coupler where the two cores are closely spaced, the distance edge-to-edge between the two cores is small (e.g. less than 5 μm apart or even less than 1 μm apart), especially relative to the waveguide height—i.e. there will be a large aspect-ratio (waveguide height divided by the edge-to-edge distance).
In a deposit-etch-deposit planar technology, a layer of core material is firstly deposited on a lower cladding layer, secondly the core layer is patterned using standard photolithographic techniques and the pattern is transferred to the core layer by etching. The pattern created during the etch step is finally covered and protected by deposition of a layer of material typically having optical characteristics as the lower cladding layer.
If one uses a deposition method that does not deposit conformably onto the underlying structures, i.e. does not deposit as fast (typically slower) on vertical faces (in a direction of growth or deposition) as on horizontal faces (i.e. parallel to a planar face of the substrate and perpendicular to a direction of growth of planar layers), problems are likely to arise in areas having large aspect-ratios. During deposition the area above the narrow opening will gradually close while leaving the volume between the two waveguides partly empty (i.e. comprising voids, so-called ‘keyholes’ or ‘air-pockets’). The deposition rate at the horizontal face at the bottom of the narrow opening is considerably lower than on horizontal faces outside the coupling region, as the material flow into the volume between the two waveguides is restricted by a shadowing effect from the waveguides themselves. Furthermore, the deposition of material grows laterally (i.e. extending from side to side) from the upper corners of the waveguides towards the central part of the coupling region which further increases the shadowing effect. The result is that a void or air pocket is created in the region between the two waveguides (cf. FIG. 22.b).
The magnitude of the exponentially decaying evanescent field tail is more or less exponentially dependent upon the refractive index difference of the core material and the surrounding material. Typically the refractive index of the material that surrounds the core will have a value only slightly smaller than that of the core, in order to create a structure that is matched to a standard optical fibre, e.g. a SMF-28 fibre. For such a fibre the core refractive index typically is around 1.450 @ 1.55 μm, whereas the cladding refractive index has a refractive index around 1.445 @ 1.55 μm, i.e. a refractive index difference of around 5·10−3. If a void is present between the two closely spaced waveguide cores of the directional coupler this void will constitute an area having a refractive index 1 (that of a vacuum), hence the refractive index difference will now be around 0.450. This will obviously make the optical coupling between the two waveguides uncontrollable and non-reproducible and thus render the component useless.
To ease the filling of the narrow space between the two waveguides in the coupling region, a multi-step process is typically applied. In such a process a layer of cladding material is deposited followed by a high-temperature treatment where the entire structure is heated to above the glass-transition temperature of the cladding material, which makes the cladding material soft. When the cladding material is soft it can flow and redistribute itself (reflow) thereby better fill the narrow spaces. This process is repeated a number of times making it possible to cover most encountered structures. However, sometimes structures/designs having aspect-ratios that prevent perfect filling of the narrow spaces are seen.
One solution to the problem of gap filling is to increase the doping level of boron and phosphorus. This will “soften” the cladding material even further and thereby promoting the gap filling. However, the higher doping concentration makes the glass less reliable and more susceptible to water. It is therefore necessary to use hermetical packaging which increases the cost of the components.
Another solution to the problem is to use other deposition processes such as flame-hydrolysis deposition (FHD) or Low Pressure Chemical Vapour Deposition (LPCVD). Both these processes have better step-coverage properties than plasma enhanced chemical vapour deposition (PECVD), but other factors such as lack of scalability, flexibility, control and automatic fabrication, etc. speak against these methods.